The invention relates generally to MR imaging and, more particularly, to a method and apparatus of parallel excitation by a transmit coil array to realize desired spatial spectra of excitation pulses.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization,” MZ, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Spatially selective excitation is widely used in MR imaging to induce transverse magnetization while limiting the size of the signal-contributing volume. Slice-selective excitation, the most commonly used excitation, confines the signal-contributing volume to a fixed slice that simplifies spatial encoding during signal acquisition to reduce data acquisition or scan time. Multi-dimensional excitation that produces localization along more than one dimension has been used to further this reduction in scan time. For example, localized spectroscopy, reduced-FOV scan of a region of interest, imaging of a target anatomy of unique shape, and echo planar imaging (EPI) with a shortened echo train length are applications usually implemented because of their support of reduced scan times. In addition, profile (flip, phase, and frequency) control across a sizeable volume with selective excitation has been exploited to improve excitation profile fidelity in the presence of B0 inhomogeneity or gradient non-linearity, and to reduce susceptibility artifacts.
Selective excitation is commonly implemented with a single transmit coil, e.g., a birdcage coil, that transmits across an entire volume and produces a relatively uniform B1 field. Highly efficient pulse algorithms have been developed for designing excitation pulses that suit such a configuration. Notwithstanding the advantages achieved by these pulse design tools, technical difficulties remain. Issues with excitation pulse duration, excitation profile accuracy, and RF power absorption (SAR) represent some of the outstanding challenges in a variety of applications. Compared to 1D excitation, flexible profile control along multiple dimensions with 2D or 3D excitation entails intensified pulsing activity and often requires powerful gradients to keep pulse duration in check. This limitation hinders applications of multi-dimensional excitation on scanners with general-purpose gradients. Substantial subject-dependency of B1 field, resulting from increased wave behavior and source-subject interaction at high frequencies, may also contribute to the difficulty of excitation profile control. An elevated rate of RF power deposition at high frequencies represents yet another factor that has a significant impact on the design and application of RF transmit modules and/or excitation pulses.
Use of adiabatic pulses represents a pulse design approach that addresses the difficulty of excitation profile control associated with B1 inhomogeneity. This approach is limited as its application has been limited to certain profiles and tends to involve high RF power. A B1-field optimization approach that aims at maximizing global B1 homogeneity addresses the control issue through transmit module improvements. Adaptation of the transmit coil geometry or the driving mechanism has been shown to reduce B1 inhomogeneity. At high frequencies, however, the capability of a field optimization approach is limited. Even with calibration-guided adjustment of driving port weights, the degree to which the spatial variation of the composite B1 field approaches a desired level is highly dependent on the characteristics of component B1 fields, and results tend to be subject to considerable residual inhomogeneity.
Another proposed solution to reduce excitation pulse length is based on a parallel excitation architecture—multiple transmit elements driven by independent drivers. Individual B1 field patterns are employed to suppress aliasing lobes arising from sampling density reduction in the excitation k-space. Aliasing sidelobes may be reduced by designing an RF pulse sequence that is specific to each transmit coil to induce spatio-temporal variations in a composite B1 field. However, discontinuities at the object boundary of practically acquired B1 maps may lead to compromises in aliasing sidelobe suppression, and measures taken to produce parallel transmit (Tx) pulses tend to produce pulses having excess energy at high spatial frequencies. This can render pulses that are sensitive to imperfections in the B1 maps as well as perturbations to the B1 mapping process.
Parallel receive (Rx) MRI, to an extent, similarly faces a significant issue of residual aliasing artifacts at low spatial resolution. Yet, while symmetry may exist to an extent between parallel Rx and parallel Tx MR imaging, each provides unique challenges that are not completely analogous to one another. In contrast to parallel Rx, where k-space coverage is typically carried out in segments (one per TR period), segmentation of a parallel excitation pulse over multiple TR periods represents an undesired compromise. Such a constraint together with the constraint of maximum length of the excitation pulse in one TR (due to, for instance, off-resonance and relaxation rates) thus limits the total coverage of the excitation k-space.
It would therefore be desirable to have a system and method capable of realizing desired excitation profiles by reducing excitation pulse length and optimizing the spatial spectra of multiple parallel Tx pulses.